Waveguide mode filter

ABSTRACT

A waveguide mode filter comprises a plurality of very thin, narrow, axially oriented resistive strips deposited on the inner surface of a dielectric lined waveguide section. The circumferential surface impedance of the strips is essentially purely reactive while the longitudinal or axial surface impedance has significant resistive and reactive components. Thus the strips present high attenuation to non-TEON modes and low attenuation to the desired TE01 modes thereby providing the same filtering function as helix waveguide while offering the manufacturing advantages of dielectric lined waveguide.

United States Patent 1 Den [ 1 May 8, 1973 [54] WAVEGUIDE MODE FILTER [75] Inventor: Chi Fu Den, Summit, NJ.

[73] Assignee: Bell Telephone Laboratories, Incorporated, Murray Hill, NJ.

22 Filed: Mar. 15, 1972 21 App]. No.2 234,867

[52] U.S. Cl. ..333/95 R, 333/98 M, 333/81 B [51] Int. Cl. ..H0lp 1/16, 1101;) 1/22 [58] Field of Search ..333/98 M, 95 R, 73 W,

[56] References Cited UNITED STATES PATENTS 2,981,907 4/1961 Bundy ..333/98 R X 3,016,502 1/1962 Unger ..333/98 M 3,078,428 2/1963 Miller ..333/95 R 3,251,011 5/1966 Unger ..333/98 M 3,275,955 9/1966 Prache ..333/95 R FOREIGN PATENTS OR APPLICATIONS 888,530 12/1943 France ..333/95 R 603,119 6/1948 Great Britain ..333/98 M OTHER PUBLICATIONS Barlow, H. E. M. A Method of Changing the Dominant Mode in a Hollow Metal Waveguide & its Application to Bends" lEE Vol. 1068 Supp. 13, 1959, pp, 100-105.

Karbowiak, A. E. Microwave Propagation in Anisotropic Waveguides IEE Vol. 103 C, 1956, pp. 139-144.

Primary Examiner-Rudolph V. Rolinec Assistant ExaminerWm. H. Punter Attorney-R. J. Guenther et al.

[57] ABSTRACT A waveguide mode filter comprises a plurality of very thin, narrow, axially oriented resistive strips deposited on the inner surface of a dielectric lined waveguide section. The circumferential surface impedance of the strips is essentially purely reactive while the longitudinal or axial surface impedance has significant resistive and reactive components. Thus the strips present high attenuation to non-TE modes and low attenuation to the desired TE, modes thereby providing the same filtering function as helix waveguide while offering the manufacturing advantages of dielec tric lined waveguide.

7 Claims, 6 Drawing Figures PATENTEDHAY 8107s SHEET 2 OF 2 FIG. 3A

FIG. 38

"ZLINE A 4 r F FIG. 4B

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to waveguide mode filters and in particular to filters utilizing thin resistive strips on the inner surface of a dielectric lining to change the propagation constants of spurious modes with respect to the desired mode of propagation.

2. Description of the Prior Art The TE circular wave mode is known to be well suited for long distance transmission of high frequency broad band signals because the attenuation of this mode decreases with increasing frequency. However, waveguides which are large enough to transmit the TE wave mode can also support other unwanted modes. Conversion and reconversion of energy between the TE mode and other modes has a degrading effect on the quality of the transmission. Con sequently, much effort has been directed toward finding ways of preventing the conversion of the TE mode into spurious modes and removing or filtering these spurious modes from the waveguide system.

Waveguides containing a thin dielectric lining have been effective in reducing the conversion of the TIE. mode into spurious modes by increasing the difference in phase constants between these modes. Dielectric lined waveguide is relatively easy and economical to manufacture because an outside-in process is used involving placing a dielectric lining in a preformed metal tube which can be made relatively accurately.

Helix waveguide is an effective-means of removing or filtering unwanted modes from a waveguide transmis sion system. However, helix waveguide is relatively difficult and expensive to manufacture. Helix waveguide is normally formed by an inside-out process involving winding a helix wire on a mandrel and subsequent encasement in a desired tube. The helix waveguide thus obtained is normally not as straight and accurate as dielectric lined waveguide which utilizes preformed metal tubing.

The use of vanes of dissipative or resistive material within the waveguide provides one alternative to helix waveguide. Such vanes can comprise separate structures which are inserted in the waveguide or which can be formed within the dielectric lining itself. The vanes primarily affect the attenuation constants and provide a distributed loading effect because of the size of the vanes with respect to the wavelengths of the signals being transmitted. Often, however, a discrete load rather than a distributed load is desirable and more effective mode filters can be obtained by controlling both the attenuation and phase constants. The vanes involving the use of separate structures are difficult to manufacture and therefore relatively costly. The formation of vanes within the dielectric lining itself offers many advantages but may not be possible where very thin dielectric linings are used.

SUMMARY OF THE INVENTION The foregoing limitations on prior art mode filters are overcome in part in accordance with this invention by a waveguide mode filter which utilizes a plurality of thin resistive strips on the inner surface of the dielectric lining. Very thin, narrow, closely spaced strips of an appropriate material are formed on the inner surface of the lining in the axial direction of the waveguide. The circumferential surface impedance of the strips is essentially reactive while the axial surface impedance has substantial resistive and reactive components. Thus the strips present high attenuation to non-TE modes and low attenuation to the desired TE mode thereby providing the same filtering function as helix waveguide while offering the manufacturing advantages of dielectric lined waveguide.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view partly broken away of a waveguide mode filter in accordance with the invention;

FIG. 2 is a schematic representation of a plane model for the filter of FIG. 1;

FIGS. 3A and 3B are respectively illustrations of the interaction of an electromagnetic wave with the wall of the mode filter and a transmission line equivalent representation thereof; and

FIGS. 4A and 4B are illustrations similar to FIGS. 3A and 38, respectively, for a plane wave having a different polarization.

DETAILED DESCRIPTION Referring now to FIG. 1 there is shown a waveguide mode filter 101 comprising a tube 2 of conductive material about the interior surface of which is a dielectric layer 4 so as to form a section of dielectric lined waveguide known in the prior art. Tube 2 can, for example, comprise a copper tube or a copper plated steel tube which can be formed very accurately and layer 4 can comprise polyethylene.

In accordance with this invention the dielectric lined waveguide section is converted into a mode filter by forming a plurality of thin, resistive strips 6 on the inner surface 5 of layer 4. Strips 6 are oriented parallel to the longitudinal axis 1 of filter 101. The thickness 7 of strips 6 is much less than the shortest free space wavelength of the frequencies being transmitted through filter 101. For example, if the shortest free space wavelength is in the order of 3 millimeters, the thickness 7 of strips 6 should be not greater than approximately 300 Angstroms. Strips 6 should appear continuous in the axial or longitudinal direction with respect to the frequencies being transmitted. This condition is essentially satisfied if the length of strips 6 is greater than 10 times the free space wavelength of any frequency of interest.

In order for the wall of filter 101 to appear macroscopically homogeneous there should be 10 or more strips for each unit of circumference equal to the shortest free space wavelength of the frequencies being transmitted. Accordingly, the spacing 8 and width 9 of strips 6 are interrelated as will subsequently become more apparent.

Strips 6 are advantageously formed from a conductive material such as silver. However, because of their small dimensions strips 6 are essentially resistive. Strips 6 can be formed on layer 4 by selective deposition or similar film-forming techniques, by removing portions of a continuous metal layer by selective etching or by other techniques which would be apparent to those skilled in the art.

The interaction of a propagating electromagnetic wave with the cylindrical wall of filter 101 can be ap proximated by a plane wave incident upon a plane surface as illustrated in FIG. 2 in which both the cylindrical coordinates r, d), as shown in FIG. l, and the local rectangular coordinates x, y, z are shown. In this illustration an electromagnetic wave represented by electric field vector 20 propagating in the direction of the z axis has a plane of incidence 21 parallel to the x-z plane, i.e., parallel to resistive strips 24. When vector 20 is polarized as shown by indicator 26 in a direction normal to plane 21, i.e., normal to strips 24, as is shown in FIG. 3A, the array of resistive strips 24 can be replaced with an equivalent impedance sheet 26 over a dielectric layer 27 having a thickness 29 and a conducting wall 28. The surface impedance of sheet 26 in the y or :1) direction, i.e., equivalent to the impedance of strips 24 in the y or (1) direction, is given by:

nu Ru 8u (l) where R and X are the resistive and reactive components, respectively, of the surface impedance. The reactive component X is capacitive in nature with respect to this polarization.

The transmission line equivalent for a wave having the foregoing polarization is shown in FIG. 3B in which Z is the wall impedance in the y direction, Z, is the surface impedance defined by equation 1 and Z is the transmission line impedance. The wall impedance can be defined by: Z Z E,,/H ([N tan pt) (R j 8u)/( y p s1 j su) where:

E is the electric field in the y direction;

H is the magnetic field in the z direction;

# is the permeability of free space;

s is the permittivity of free space;

6, is the dielectric constant of dielectric layer 27;

A is the free space wavelength of the frequency of interest; and

t is the thickness 29 of dielectric layer 27.

Likewise, when the incident electric field is polarized in the plane of incidence 21, i.e., parallel to strips 24 as illustrated in FIG. 4A by indicator 30, the array of strips 24 can be replaced by an equivalent impedance sheet 31 having a surface impedance in the z direction of:

n2 j az (3) where R and X are the resistive and reactive components, respectively, and the reactive component X is inductive in nature for this polarization.

The transmission line equivalent for this polarization is shown in FIG. 4B in which Z is the wall impedance in the z direction, Z is the surface impedance defined by equation (3) and 2, is the transmission line impedance. This wall impedance is given by:

where:

E, is the electric field in the z direction; H is the magnetic field in the y direction;

N, is 1/6.

and all other quantities have been previously defined.

The loss or attenuation ofa TE mode through filter 101 is proportional to the magnitude of the real part of Z whereas the attenuation of a non-TE mode is proportional to the magnitude of the real part of Z provided the following conditions are met:

i yi i zl o/ o where D is the waveguide diameter of filter 101 and all other parameters have been previously defined. Thus for a high degree of mode filtering, i.e., effective suppression of spurious non-TE modes simultaneous with low attenuation of the TE mode, the width 23 and spacing 25 of strips 24 are chosen, subject to the previously mentioned constraint of macroscopic homogeneity so that:

X R N tan pt; and (7) X R g N tanpt. (8)

Under these constraints equations (2) and (4) can be respectively approximated by:

2,, jN tan pt; and 9 Z, jR, N tan pz/jN, tan pz R 10) Hence the real part of Z is approximately zero and the attenuation of the TE mode is low. On the other hand, Z has substantial real components and the undesired non-TE modes are effectively suppressed. The argument of 2 can be controlled by control of such parameters as the width 23 and spacing 25 of strips 24, the resistivity of strips 24, and the thickness of dielectric layer 27 thereby to obtain the desired relative magnitudes of the resistive and reactive components of Z for any particular situation.

While the invention has been described with respect to a specific embodiment thereof, it is to be understood that various modifications thereto might be made by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

I. A mode filter for passing the TE mode and suppressing wave modes having longitudinal current components comprising, in combination:

a section of conductive pipe having a dielectric lining around the inner surface thereof; and

a plurality of thin resistive strips mounted on the inner surface of said lining and extending in the direction of the longitudinal axis of said section, said strips having dimensions and spacings such that the circumferential component of wall impedance of said filter comprises essentially a reactive component and the longitudinal component of wall impedance of said filter comprises both reactive and resistive components so that said TE mode is passed while said wave modes having longitudinal current components are suppressed.

2. Apparatus in accordance with claim 1 wherein the length of said strips is at least as great as times the free space wavelength of the frequency of said modes being transmitted through said filter.

3. Apparatus in accordance with claim 1 wherein said strips have a width and spacing such that there are at least 10 of said strips around each unit of the inner periphery of said lining equal to the free space wavelength of the highest frequency of said modes being transmitted.

4. Apparatus in accordance with claim 1 wherein said filter has circumferential and longitudinal components of wall impedance respectively defined by:

Z,, circumferential component of wall impedance; Z, longitudinal component of wall impedance;

[4, permeability of free space;

6,, permittivity of free space;

e, dielectric constant of said lining;

A the free space wavelength of said modes being transmitted;

t= the thickness of said lining; and

R, the resistive component of surface impedance of said strips.

5. Apparatus in accordance with claim 1 wherein said strips have a thickness no greater than 300 Angstrorns.

6. Apparatus in accordance with claim 1 wherein said strips comprise strips of silver formed on said lining.

7. A mode filter for passing the TE mode and suppressing non-circular wave modes comprising, in combination:

a section of conductive pipe having dielectric lining around the inner surface thereof; and

a plurality of thin resistive strips mounted on the inner surface of said lining and extending in the direction of the longitudinal axis of said section. 

1. A mode filter for passing the TE01 mode and suppressing wave modes having longitudinal current components comprising, in combination: a section of conductive pipe having a dielectric lining around the inner surface thereof; and a plurality of thin resistive strips mounted on the inner surface of said lining and extending in the direction of the longitudinal axis of said section, said strips having dimensions and spacings such that the circumferential component of wall impedance of said filter comprises essentially a reactive component and the longitudinal component of wall impedance of said filter comprises both reactive and resistive components so that said TE01 mode is passed while said wave modes having longitudinal current components are suppressed.
 2. Apparatus in accordance with claim 1 wherein the length of said strips is at least as great as 10 times the free space wavelength of the frequency of said modes being transmitted through said filter.
 3. Apparatus in accordance with claim 1 wherein said strips have a width and spacing such that there are at least 10 of said strips around each unit of the inner periphery of said lining equal to the free space wavelength of the highest frequency of said modes being transmitted.
 4. Apparatus in accordance with claim 1 wherein said filter has circumferential and longitudinal components of wall impedance respectively defined by:
 5. Apparatus in accordance with claim 1 wherein said strips have a thickness no greater than 300 Angstroms.
 6. Apparatus in accordance with claim 1 wherein said strips comprise strips of silver formed on said lining.
 7. A mode filter for passing the TE01 mode and suppressing non-circular wave modes comprising, in combination: a section of conductive pipe having dielectric lining around the inner surface thereof; and a plurality of thin resistive strips mounted on the inner surface of said lining and extending in the direction of the longitudinal axis of said section. 